As the catalyst for purifying nitrogen oxide (NOx) contained in an exhaust gas, a NOx storage-reduction catalyst (hereinafter, referred to as NSR catalyst) of absorbing and removing NOx contained in the exhaust gas is known. The NSR catalyst has an NOx storage/release action of absorbing NOx in a supplied exhaust gas by a storage material in the form of nitrate when the air-fuel ratio of the supplied exhaust gas is leaner than a predetermined value (typically, a theoretical air-fuel ratio), i.e., in an oxygen excess atmosphere, and releasing the absorbed NOx when the air-fuel ratio of the supplied exhaust gas is richer than the predetermined value, i.e., in an oxygen deficient atmosphere, allowing it to react with hydrocarbon (HC) with the aid of a catalyst such as platinum and be reduced to N2 and discharged into outside air.
The reducing ability of the NSR catalyst tends to deteriorate due to poisoning by a sulfur component contained in the exhaust gas, abnormal heat generation, etc. Accordingly, various techniques for diagnosing deterioration of a catalyst have been developed so as to detect deterioration of the NSR catalyst and apply an appropriate treatment.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2013-019401 describes a catalyst deterioration judging system where a temperature sensor for detecting the temperature of an NSR catalyst is disposed, the air-fuel ratio of an exhaust gas passing through the NSR catalyst is controlled to a rich air-fuel ratio of 14 or less, and when the temperature rise amount of the NSR catalyst here is less than a threshold value, the NSR catalyst is determined as being deteriorated.